Ethernet cable (Was: Sun3 valuations?)
paulkoning at comcast.net
Tue Jan 23 13:07:47 CST 2018
> On Jan 23, 2018, at 11:10 AM, Bill Gunshannon via cctalk <cctalk at classiccmp.org> wrote:
> If you didn't locate the transceivers on those black marks you would
> have had terrible performance as that affects collisions. Timing (among
> other things like grounding) was very important with that version of
> ethernet hardware.
Yes, the purpose of the marks is to make the collision mechanism reliable.
Ethernet does not have any critical timing; collisions do not depend on timing. The black stripes on official Ethernet cable exists for a different reason: to get you to place the taps at positions that are NOT round multiples of a quarter wavelength. The reason: a tap is a (small) impedance bump, which causes reflections on the cable. If you have a lot of taps and they are spaced multiples of a wavelength apart, those reflections will combine to produce a large reflection, which if you're unlucky will look like a collision. If you pick the correct spacing, the reflections from the various taps are spread out across time and don't combine, so none of them add up to a strong enough pulse to be seen as a collision.
This is clearly stated in the Ethernet V2 spec, section 7.6.2:
> Coaxial cables marked as specified in 18.104.22.168.6 have marks at regular 2.5 meters spacing; a transceiver may be placed at any mark on the cable. This guarantees both a minimum spacing between transceivers of 2.5 meters, as well as controlling the relative spacing of transceivers to insure non-alignment on fractional wavelength boundaries.
Reading between the lines, it's clear you could ignore those marks and get away with it in many cases. Low tap count, for example. Other positioning that meets the "non-alignment" intent. But for large installations, using the marks ensures that you stay out of trouble.
The need to have a transmission line with controlled reflections is also why the cable is required to be terminated with accurate terminating resistors, at both end points (but not at any other point :-) ) and why splices are made with constant impedance connectors (N connector barrels).
Apart from the marks, the 10Base5 cable is pretty ordinary. It's not exactly RG-8/U but it is not all that differen either, and if the diameter is close enough something like RG-8/U would make an acceptable substitute.
The same sort of considerations could apply to 10Base2, but there things are not as strict because the cable is shorter and the station count is significantly lower (max of 30). So the spec simply states that stations should be at least 1/2 meter apart, and that there must not be a significant stub (more than a few centimeters) between the T connector and the transceiver electronics.
If you build with transmission line design rules in mind, you can make Ethernet buses out of cable of your choice, so long as it's 50 ohms and good quality components are used throughout. You can, for example, splice 10Base5 to 10Base2 (with a barrel, not a T) if you follow the more restrictive of the two configuration rules.
More information about the cctalk